1. Field of the Invention
The present invention relates to a wireless power supply technique.
2. Description of the Related Art
In recent years, in order to supply electric power to an electronic device, contactless power transmission (which is also referred to as “contactless power supply” or “wireless power supply”) has begun to come into commonplace use. In order to advance the compatibility of products between manufacturers, the WPC (Wireless Power Consortium) has been organized, and the WPC has developed the Qi standard as an international standard.
A wireless power supply that conforms to the Qi standard makes use of electromagnetic induction between a transmission coil and a reception coil. FIG. 1 is a diagram showing a configuration of a wireless power supply system 10 that conforms to the Qi standard. The power supply system 10 includes a power transmitter 20 (TX, Power Transmitter) and a power receiver 30r (RX, Power Receiver). The power receiver 30r is mounted on an electronic device such as a cellular phone terminal, smartphone, audio player, game machine, tablet terminal, or the like.
The power transmitter 20 includes a transmission coil (primary coil) 22, a driver 24, a controller 26, and a demodulator 28. The driver 24 includes an H-bridge (full-bridge) circuit or otherwise a half-bridge circuit. The driver 24 applies a driving signal S1, and specifically a driving signal S1 in the form of a pulse signal, to the transmission coil 22 such that a driving current flows through the transmission coil 22, thereby generating an electric power signal S2 in the form of an electromagnetic field signal. The controller 26 integrally controls the overall operation of the power transmitter 20. Specifically, the controller 26 controls the switching frequency of the driver 24 or otherwise the duty ratio of the switching of the driver 24 so as to adjust the electric power to be transmitted.
In the Qi standard, a protocol is defined for communication between the power transmitter 20 and the power receiver 30r, which enables information transmission from the power receiver 30r to the power transmitter 20 via a control signal S3. The control signal S3 is transmitted from a reception coil 32 (secondary coil) to the transmission coil 22 in the form of an AM (Amplitude Modulation) modulated signal using backscatter modulation. The control signal S3 includes electric power control data (which will also be referred to as a “packet”) which controls an amount of electric power to be supplied to the power receiver 30r, and data which indicates the identifying information for the power receiver 30r. The demodulator 28 demodulates the control signal S3 included in the current or otherwise the voltage applied to the transmission coil 22. The controller 26 controls the driver 24 based on the power control data included in the control signal S3 thus demodulated.
The power receiver 30r includes the reception coil 32, a rectifier circuit 34, a smoothing capacitor 36, a charger circuit 38, a controller 40, and a modulator 42. The reception coil 32 receives the electric power signal S2 from the transmission coil 22, and transmits the control signal S3 to the transmission coil 22. The rectifier circuit 34 and the smoothing capacitor 36 rectify and smooth a current S4 induced at the reception coil 32 according to the electric power signal S2, thereby converting the current S4 into a DC voltage.
Using electric power supplied from the power transmitter 20, the charger circuit 38 charges a secondary battery 50. The charger circuit 38 includes a converter that steps up or otherwise steps down the DC voltage VRECT. The charger circuit 38 supplies the DC voltage thus stepped up or stepped down to the controller 40 and other components such as the secondary battery 50.
The controller 40 monitors the electric power supplied to the secondary battery 50, and generates electric power control data to be used to control the amount of electric power supplied from the power transmitter 20. The modulator 42 modulates the control signal S3 including the electric power control data so as to modulate the coil current that flows through the reception coil 32, thereby modulating the coil current and coil voltage applied to the transmission coil 22.
FIG. 2 is a flowchart (state transition diagram) showing the power supply system 10 shown in FIG. 1. Description will be made assuming that the following five phases are defined.                φ1: Selection phase        φ2: Ping phase        φ3: Identification and configuration phase        φ4: Negotiation phase        φ5: Power transfer phase        
In the selection phase φ1, the power transmitter 20 detects the presence or absence of the power receiver 30r. Specifically, the power transmitter 20 applies a current to the transmission coil 22 at predetermined intervals (e.g., at intervals of 5 ms). The current that flows through the transmission coil 22 varies depending on whether or not the reception coil 32 exists in the vicinity of the transmission coil 22. The power transmitter 20 uses this mechanism to judge whether or not the power receiver 30r is placed on a charging stand. Such a judgment operation will be referred to as the “analog ping operation”.
When the power receiver 30r is detected, the phase transits to the ping phase φ2. In the ping phase φ2, the power transmitter 20 executes a digital ping operation, and waits for a response from the power receiver 30r. In the digital ping operation, the electric power to be transmitted is maintained at a constant level, and the power receiver 30r operates using the electric power signal S2 received from the power transmitter 20 as a power supply.
In the subsequent identification and configuration phase φ3, the power transmitter 20 identifies the power receiver 30r, and performs settings with respect to the electric power to be transmitted or the like.
In the negotiation phase φ4, the electric power to be transmitted or the like is reconfigured. In the power transfer phase φ5, electric power is transferred based on the information thus configured.
When the secondary battery 50 becomes the full charge state on the power receiver 30r side, the controller 40 generates an EPT (End of Power Transfer) packet, and transmits the EPT packet thus generated to the power transmitter 20.
After the power transmitter 20 receives the EPT packet, the power transmitter 20 stops the power supply operation. However, a protocol to automatically restart the charging operation is not defined. That is to say, the power receiver 30r has the potential to run out of battery charge even if the power receiver 30r remains placed on a charging stand.